This invention relates generally to a power line communication system and more particularly, to a power line communication system for communicating commands and data on a power line.
Geometric harmonic modulated (GHM) signals for power line carrier applications consist of a set of carriers, each carrier being modulated by the same data. For example, eight carriers are evenly spaced in the frequency band between 5 kHz and 12 kHz yielding a GHM signal s(t) characterized by the relationship:
si(t)=sin(2xcfx80xc2x75010t)xe2x88x92sin(2xcfx80xc2x75610t)+sin(2xcfx80xc2x76210t)+sin(2xcfx80xc2x76810t)xe2x88x92sin(2xcfx80xc2x77410t)+sin(2xcfx80xc2x78010t)+sin(2xcfx80xc2x78610t)+sin(2xcfx80xc2x79210t).
The eight carriers of the above characterized signal are positioned at selected frequencies of 5010 Hz, 5610 Hz, 6210 Hz, 6810 Hz, 7410 Hz, 8010 Hz, 8610 Hz, and 9210 Hz. The signs of the eight sinusoidal carriers are selected according to a Shapiro-Rudin sequence to keep the signaling crest factor low.
Typical GHM signals such as the one described above utilize frequencies above about 5 kHz to avoid the large spectral components of the 60 Hz power signal transmitted on most power lines. However, use of GHM signals at typical frequencies has attendant disadvantages. First, the efficiency of transmission of the GHM signal through distribution transformers drops off as the GHM signals increase in frequency. This is because the transformer acts like a low pass filter. In addition, typical GHM signals are severely attenuated by capacitor banks.
The present invention is a communication system for communicating on a power line comprising a transmitter adapted to provide a GHM signal which is lower in frequency than the typical GHM signal such that the GHM signal is capable of being transmitted through distribution transformers and capacitor banks. The GHM signal is combined with a power signal for transmission with the power signal. The system further comprises a receiver configured to remove an estimate of the power signal from the combined signal and to provide the GHM signal at an output. The GHM receiver includes an estimator for generating an estimated power signal, a summer for subtracting the estimated power signal from the signal transmitted on the power line to segregate the transmitted GHM signal si(t), an analog to digital converter coupled to an output of the summer, and a demodulator coupled to an output of the analog to digital converter.